1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a program thereof for handling a copy-forgery-inhibited pattern image.
2. Description of Related Art
Presently, there is special paper called forgery-proof paper. The forgery-proof paper has a character string such as “copy” embedded therein, which a person cannot see at a glance. On a copy obtained by making a copy of a forgery-proof paper, the embedded character string appears. Accordingly, an original document using such forgery-proof paper can be easily distinguished from a copy thereof. In addition, this makes it possible to discourage a person who is attempting forgery from using the copy of the document.
With such an effect, the forgery-proof paper has been used for preparing certificates of residence and forms. However, the forgery-proof paper has a problem of entailing higher cost than plain paper. In addition, it has a problem of causing only the character string embedded during the production of the forgery-proof paper to appear on a copy. In such circumstances, a new technique capable of achieving an effect similar to the forgery-proof paper has attracted attention recently (see Japanese Patent Laid-Open No. 2001-197297). It combines source document image data formed by using a computer with copy-forgery-inhibited pattern (sometimes referred to as “copy check pattern”) image data, and outputs the image data obtained by the combination, including the copy-forgery-inhibited pattern, to plain paper.
Into the copy-forgery-inhibited pattern image data, a prescribed character string or the like is embedded. Thus, on a copy obtained by copying a document containing of the image data including the copy-forgery-inhibited pattern, the character string appears in the same manner as in the case of using the forgery-proof paper.
The technique uses plain paper. Accordingly, it has an advantage of being able to create an original cheaper than using the forgery-proof paper. In addition, the technique can generate new image data with a copy-forgery-inhibited pattern every time an original is created. Accordingly, the technique can freely set the copy-forgery-inhibited pattern color of the copy-forgery-inhibited pattern image data and can set a character string to be embedded into the copy-forgery-inhibited pattern.
Incidentally, the copy-forgery-inhibited pattern image data consists of a “surviving” region and a “disappearing” region (or a region that becomes thinner than the “surviving” region) on a copy. The density levels of the two regions are nearly equal on the original. Thus, it is difficult for human eyes to perceive that a character string such as “COPY” is embedded. Here, the term “surviving” means that the image in the original reappears precisely on the copy article. On the other hand, the term “disappearing” means that it is difficult for the image in the original to reappear on the copy.
From this point forward, the “surviving” region on the copy is referred to as a “latent image portion”, and the “disappearing” region (or a region that becomes thinner than the “surviving” region) on the copy is referred to as a “background portion”.
FIG. 14 is a diagram showing a state of dots in the copy-forgery-inhibited pattern image data. In FIG. 14, a dot assembled (i.e., clustered) region is a latent image portion, and a dot dispersed region is a background portion. The dots in the two regions are generated by different halftone dot processing or by different dithering processing. For example, the dots in the latent image portion are generated by halftone dot processing with a small Lines Per Inch (LPI), and the dots in the background portion are generated by halftone dot processing with a large Lines Per Inch (LPI). Alternatively, the dots in the latent image portion are generated using a dot concentrated-type dithering matrix, and the dots in the background portion are generated using a dot dispersed-type dithering matrix.
Incidentally, the reproducibility of a copying machine depends on the input resolution or output resolution of the copying machine. Accordingly, the reproducibility of the copying machine has a limit. Consider a case where the dots in the latent image portion of the copy-forgery-inhibited pattern image data are made greater than the dots reproducible by the copying machine and the dots in the background portion are made smaller than the dots reproducible by the copying machine. In such a case, although the dots in the latent image portion reappear on a copy in general, the dots in the background portion are difficult to make reappear. As a result, the latent image portion reappears denser than the background portion on the copy article. From now on, a state that an embedded character string emerges because the latent image portion reappears denser than the background portion on the copy article is referred to as visualizing.
FIGS. 15(a) and 15(b) are drawings showing the visualizing. These drawings schematically show that, although the assembled dots (larger dots) reappear on the copy, the dispersed dots (smaller dots) do not reappear precisely on the copy.
As for the copy-forgery-inhibited pattern image data, it is not limited to the foregoing configuration, but can have any configuration that renders visible (visualizes) character strings like “COPY”, symbols and patterns in such a manner that a person can recognize them on the copy. In addition, even if a character string such as “COPY” is shown in white on the copy, it can be said that the copy-forgery-inhibited pattern image data achieves its object. In this case, it goes without saying that the region of “COPY” is referred to as a background portion.
The constituent elements of the copy-forgery-inhibited pattern are a background portion and a latent image portion, and it is important that the two types of regions are represented on the original at nearly the same density level. For example, Japanese Patent Laid-Open No. 2006-229316 describes a method of compensating for the dot reproducibility of an image forming apparatus due to long-term deterioration. More specifically, a technique is described which carries out calibration of the latent image portion and background portion of the copy-forgery-inhibited pattern using screens with various Lines Per Inch (LPI), and switches the screen between the background portion and latent image portion of the copy-forgery-inhibited pattern.
However, there is a problem unsolvable by switching the Lines Per Inch (LPI) of the screen. The problem is that the density level of the copy-forgery-inhibited pattern changes when outputting the image data including the copy-forgery-inhibited pattern (combined image data) to paper after rotating it.
In the image forming apparatus, according to a paper feed direction or a paper output direction, or according to user instructions, the image data including the copy-forgery-inhibited pattern (combined image data) is sometimes output after having been rotated. Naturally, the latent image pattern (dot pattern of the latent image portion) and background pattern (dot pattern of the background portion) combined in the image data with the copy-forgery-inhibited pattern are also rotated and output. In the course of this, the latent image pattern and the background pattern (the two patterns are together referred to as a “copy-forgery-inhibited pattern”) are affected by characteristics of the image forming apparatus at the time of image forming, which differ from those when they are not rotated. As a result, the density levels when they are output to paper differ from each other.
For example, when the image forming apparatus is an electrophotography system, the difference in characteristics results from a fact that the potential state on the photoconductive drum varies depending on whether the rotation is made or not, because of differences in the transient response characteristics or in the spot diameter of the laser of the image forming apparatus, as will be described below.
FIG. 16 is a schematic view of that.
FIG. 16A shows that because of the transient response characteristics of the laser, the laser scanning is carried out in a different manner at the time when the digital image data consists of 1×2 pixels, or 2×1 pixels obtained by rotating it. The output of the laser cannot follow the digital data directly, but takes some time in response. For 1×2 pixels, the laser scanning of one pixel is carried out twice, whereas for 2×1 pixels, the laser scanning of two pixels is carried out once. Because of the transient response characteristics of the laser, the scanning of one pixel is not equal to half the scanning of two pixels. Thus, the potential state on the photoconductive drum changes, and even if they are digital dots of the same size, their density level on the output document differs from each other.
FIG. 16B shows that, because of the difference in the spot diameter of the laser, the laser scanning is carried out in a different manner for the 1×2 pixels of the digital image data and for the 2×1 pixels obtained by rotating it. The spot diameter of the laser differs in the main scanning direction and subscan direction, and has an elliptical shape as shown in FIG. 16B. Here, for the 1×2 pixels, the laser scanning for one pixel is carried out twice, and for 2×1, the laser scanning for two pixels is carried out once. Because of the difference in the spot diameter of the laser, the two pixels in the subscan direction and the two pixels in the main scanning direction are not irradiated with a laser spot of the same shape. Thus, the potential state on the photoconductive drum differs, and even if they are digital dots of the same size, their density level on the output document differs from each other.
As for causes for having different characteristics due to rotation, there are various causes other than that described above, such as a timing difference in turning on and off of the laser in the subscan direction. As a result, the problem of having a different density level between the latent image and the background of the copy-forgery-inhibited pattern image arises depending on whether the copy-forgery-inhibited pattern is output without rotation or output after rotation. This is because the dot patterns differ between the latent image portion and the background portion, and hence they are subjected to different effects.
As for the copy-forgery-inhibited pattern image, the density level of the latent image portion and the density level of the background portion should be equal when printed on a sheet. Accordingly, the copy-forgery-inhibited pattern image data is formed after undergoing density level adjustment (calibration) in such a manner that the respective density levels (the density level of the background portion and the density level of the latent image portion) have the same level when printed on a sheet.
However, as for the copy-forgery-inhibited pattern image data which is generated after the calibration performed, in such a manner that the latent image and the background have the same density level at a particular image rotation angle, if the image is further rotated and printed on a sheet, the density level of the latent image portion and the density level of the background portion have a level difference. This can sometimes damage the function of the copy-forgery-inhibited pattern.